Plant Genome II Conference
Town & Country Conference Center, San Diego, CA, January, 1994.
PG-II: QTL AFFECTING SOYBEAN CYST-NEMATODE RESISTANCE
QTL AFFECTING SOYBEAN CYST-NEMATODE RESISTANCE.
D.M. Webb 1, B.M. Baltazar 1, A.P. Rao-Arelli 2, J. Schupp 3, P.
Keim 3, K. Clayton 1, A.R. Ferreira 3, T. Owens 1, and W. D.
Beavis 1, 1 Pioneer Hi-Bred Int., P.O. Box 1004 Johnston, IA
50131; 2 Dept. Agronomy, 117 Curtis Hall, Univ. Missouri,
Columbia, MO 65211; 3 Dept. Biology, NAU Box 5640, Northern Ariz.
Univ., Flagstaff, AZ 86011.
Breeding soybean for resistance to soybean cyst nematode
(SCN) is difficult, but necessary to counter seed yield losses in
SCN-susceptible cultivars grown on SCN-infected fields. The
soybean plant introduction PI437654 has resistance to more SCN
race-isolates than any other soybean tested. We genetically
mapped three resistance loci in PI437654 by co-segregation
analysis between RFLP markers and SCN race-3 cyst scores among
296 F6:7 recombinant-inbred lines from the cross PI437654/BSR101.
All three loci were needed for complete resistance to SCN race 3.
Though highly variable, on average, lines with any two of the
three resistance loci had fewer cysts than those with any one
resistance locus. However, lines with any one resistance locus
had about the some number of cysts as lines with no resistance
loci. Additional markers closely linked to two of the three
resistance loci were found by bulk-segregant analysis. We
estimated the effects of population sample size in mapping these
three loci. Sub-populations of 50, 100, 150, 200, and 250 lines
were randomly sampled 50 times each from the 296-line population
and used in QTL analyses. All three resistance loci were
detected in approximately 0, 20, 60, 98, and 100% of the
sub-populations of different sizes, respectively. We consider a
minimum population size of 200 lines and markers no more than 40
cM apart is necessary to reliably map loci for this trait.
Indirect selection using genetic markers closely linked to these
three resistance loci may be more effective than direct selection
for identifying breeding lines resistant to SCN race 3.
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